[ubuntu-us-mi] failed boot and Ubuntu cheatcodes?

[Robert Citek]

How can I boot into some kind of sub-rescue mode in order to recover
from an error in my /etc/fstab?

I did the following:

sudo true && {
sudo dd if=/dev/zero of=/just-for-fun bs=1M count=100
# sudo mkfs.ext3 -F /just-for-fun
sudo mkdir /media/jff
echo "/just-for-fun /media/jff ext3 loop 0 1" | sudo tee -a /etc/fstab
sudo reboot
}

Notice that I forgot to uncomment the mkfs.ext3 command.  Now, when I
try to boot, Ubuntu complains that it can't mount the filesystem.
That's not a big deal except that it I haven't figured out a way to
boot into some kind of rescue mode that drops me to a shell, even a
busybox shell, to let me fix the /etc/fstab.  Not even emergency mode
(-b) works.  The only way I that has worked is to boot with some other
media, e.g. another OS, LiveCD, or LiveUSB.

Knoppix has some pretty cool cheatcodes that pause the boot process at
different stages[1] so that you can fix things before moving on.  Does
Ubuntu have anything like that?

[1] http://www.knoppix.net/wiki/Cheat_Codes#Tips.26Tricks

Regards,
- Robert